The most common process for producing acrylic acid is a two-stage gas-phase catalytic oxidation process, which is widely used industrially. This process includes a first reaction of producing acrolein from propylene by gas-phase catalytic oxidation and a second reaction of producing acrylic acid from acrolein by gas-phase catalytic oxidation. Conventionally, for conducting such a reaction, there have been proposed roughly two methods: a method using two reactors, that is, a first reactor filled with a catalyst suitable for a first reaction (hereinafter referred to as a “first catalyst”) and a second reactor filled with a catalyst suitable for a second reaction (hereinafter referred to as a “second catalyst”); and a method using one reactor which includes a first reaction zone filled with the first catalyst and a second reaction zone filled with the second catalyst.
A process for producing acrylic acid from propylene by two-stage gas-phase catalytic oxidation has problems such as obstruction of a reaction tube induced by sublimates from the first reaction, more specifically, a molybdenum component sublimed from a molybdenum-containing oxide catalyst that is generally used as the first catalyst, or high-boiling substances such as terephthalic acid by-produced in the first reaction, and increase of pressure loss. Various proposals have been made about a process for producing acrylic acid more efficiently by remedying these problems.
For example, Patent Literature 1 discloses a process for producing acrylic acid from propylene by two-stage gas-phase catalytic oxidation, in which the first catalyst and the second catalyst are respectively filled into each of two reactors, that is, a primary reactor and a secondary reactor, and a rod-like or plate-like insertion object is inserted into a gas inlet space of the second catalyst so that the void ratio in the tube is 40% or more and 90% or less.
Patent Literature 2 discloses a process for producing acrylic acid using one fixed-bed multitubular heat-exchanging reactor. In Patent Literature 2, the first catalyst and the second catalyst are respectively filled into a lower part and an upper part of each of reaction tubes of the reactor to form a first catalyst layer and a second catalyst layer, and an inert substance is filled into the space between the first catalyst layer and the second catalyst layer so that the void ratio is 40% or more and 99.5% or less. Here, the inert substance layer is provided so as to have a sufficient length for cooling a reaction gas from the first catalyst layer to a temperature suitable for introducing into the second catalyst layer, and further, both the catalyst situated at an upper end part of the first catalyst layer and the catalyst situated at a lower end part of the second catalyst layer are disposed at positions where they are not substantially affected by heat from a partition plate. Acrylic acid is produced by introducing a raw material gas containing propylene from lower parts of the reaction tubes to pass through as an upward flow.
Patent Literature 3 discloses a method in which, upon conducting a gas-phase catalytic oxidation reaction, a treatment agent for removing organic matters and/or carbides is disposed at an upstream side of a gas-phase oxidation catalyst layer relative to the gas flow, and the treatment agent is exchanged at a frequency of at least once a year.
Patent Literature 4 discloses, in Example 5, that two-stage gas-phase catalytic oxidation is conducted using one multitubular heat-exchanging reactor by introducing a raw material gas containing propylene from a top of each of reaction tubes of the reactor. In Patent Literature 4, into each reaction tube of the reactor, the second catalyst is firstly filled to form a second catalyst layer, then alundum for cooling a reaction gas is filled on the second catalyst layer, and finally the first catalyst is filled on the alundum layer to form a first catalyst layer. And, the two-stage gas-phase catalytic oxidation is conducted by introducing a raw material gas containing propylene from the top of the reaction tube.
Patent Literature 5 discloses a process for producing unsaturated carboxylic acid by introducing unsaturated aldehyde into a catalyst layer filled with an uniform mixture of a catalyst containing molybdenum and vanadium, and Raschig rings made of a metal that have a bulk volume of 0.3 times to 3.5 times that of the catalyst and a filling density of 0.5 kg/l to 1.5 kg/l, to perform gas-phase catalytic oxidation.